1. Field of the Invention
The present invention relates to a method for preparing block copolymers of propylene and more particularly, to an improved method for preparing block copolymers of propylene in which an apparatus for polymerization comprising a countercurrent type washing column disposed between a polymerizer for substantially homopolymerizing propylene and a polymerizer for copolymerizing ethylene and propylene.
2. Prior Art
There have widely been carried out methods for preparing isotactic polypropylenes which comprise first substantially homopolymerizing propylene and then copolymerizing ethylene and propylene for the purpose of improving its impact resistance, in particular, that at a low temperature. On the other hand, there have been many attempts to improve the balance between the impact resistance and rigidity of isotactic polypropylenes.
In general, the balance between the impact resistance and rigidity of isotactic polypropylenes depends on the weight ratio of portions formed through the substantial homopolymerization of propylene to those formed by the copolymerization of ethylene and propylene. The impact resistance thereof is further influenced by the molecular weight of the copolymerized portions and the ratio of monomers from which copolymerized portions are derived exerts influence on the rigidity of the resulting polymer. Therefore, it is necessary to change the ratio of monomers to be copolymerized and the molecular weight of the copolymerized portions by, for instance, carrying out multi-stage polymerization in response to physical properties required of the polymer. To solve this problem, the inventors of this invention had already proposed a method comprising a combination of a continuous polymerization as a first atep and a batch polymerization as a second step, in which the effectiveness of the bulk polymerization is utilized and which provides block copolymers having excellent balance between the impact resistance and the rigidity thereof (see, for instance, U.S. Pat. No. 4,751,265). However, this method suffers from some problems. For instance, it requires the use of an excess of organoaluminum compounds or the like; the unit of catalyst such as an organoaluminum compound is low; if a method for purifying the resultant polymer comprising simply evaporating unreacted monomers after the polymerization to remove the same is adopted, the amount of the catalyst remaining in the polymer becomes large; and if a reaction in which the reaction ratio of ethylene is large is carried out in a later stage of the batch polymerization process, conditions for recovery and reuse of ethylene are greatly restricted.